Why is it so important that your customers know what your competitive advantage is? Because it's what distinguishes you from everyone else in your market.
Customers want to know what's in it for them. So be plain about it. Highlight the things that you do better than your competition.
Is your product or service a time saver? Is it convenient or is your business in a handy location?
Do you offer immediate service? Can you give customers a quick answer if they call?
Are you more reliable than your competitors? Do you make a pledge to arrive when you say you're going to?
Do you offer superior customer service? Will you go the extra mile to make sure your customer gets exactly what they want?
The best competitive advantages are the things that benefit your customer in terms of time savings, money savings, or making them feel like they're your best, most valuable customer.